This invention relates to an improvement in the drying of gas streams. More particularly, it relates to reducing the absorption of aromatics and volatile organic hydrocarbons (VOC's) in a dehydrating solvent and correspondingly, reducing the emission of absorbed aromatics as VOC's when the solvent is regenerated.
Gases generally contain varying amounts of water vapor. The presence of the water vapor is undesirable as the water vapor can result in corrosion of pipes and cause corrosion of and stoppages in valves and fittings in gas transmission systems. The presence of water is also troublesome during combustion and while being transported during freezing weather, and during transport at temperatures above freezing, can result in the formation of gas hydrates.
A common method for removing moisture from gas streams, such as natural gas, is to use a glycol dehydration unit. In such a unit, the wet gas is contacted with a lean glycol in an absorbent step to remove the water. The glycol commonly used is triethylene glycol and to a lesser extent other glycols such as diethylene glycol or ethylene glycol. The rich glycol (glycol containing the water) is then passed to a reconcentration or regeneration process in which the absorbed water is removed, thereby enabling reuse of the glycol.
During the absorption step, the glycol absorbs some of the hydrocarbons in the gas. Glycol has a high affinity for aromatic compounds, such as benzene, toluene and xylene (BTEX), so the glycol removes the BTEX as well as other aliphatic volatile organic hydrocarbons. Upon heating the glycol to remove the water, the BTEX and VOC's are lost as emissions. The loss of the aromatic and aliphatic hydrocarbons from gases, such as natural gas, through emissions is a loss of valuable hydrocarbons which could be sold for their BTU value. In addition, depending upon the local regulations, such emissions may be viewed as a source of air pollution and would need to be treated and disposed of by costly means, such as combustion or condensation, to prevent these compounds from entering the air.
To reduce the absorption of aromatics in a glycol during gas dehydration, U.S. Pat. No. 5,462,584 discloses the use of either diphenylmethane or 1,1-diphenylethane in either a glycol alone or a glycol with potassium acetate or potassium formate which allows more of the aromatics to remain with the gas for subsequent combustion with the gas.
Notwithstanding the improved dehydration of natural gas with the use of glycols and diphenylmethane or 1,1-diphenylethane, there remains a need for an effective process for treating a gas stream.